1. Technical Field
The present invention relates to a fixing unit and an image forming apparatus with the fixing unit. More specifically, the present invention relates to a fixing unit installed in an image forming apparatus, such as a copier, a facsimile, a printer, etc., employing an electrophotography system.
2. Related Art
At present, various types of image forming apparatuses, such as copiers, facsimiles, printers, etc., employing an electrophotography system have been developed and are widely known. The image forming process employed in the image forming apparatuses is realized in such a manner that an electrostatic latent image is initially formed on a surface of a photosensitive drum serving as an image carrier. The electrostatic latent image on the photosensitive drum is subsequently developed and rendered visible by toner, etc., serving as developer. The thus developed image is subsequently transferred and borne on a recording medium (hereinafter sometimes referred to as a paper sheet, a recording sheet, or a transfer sheet) by a transfer unit. The toner image thus developed and borne on the recording medium is subsequently fixed by a fixing unit using pressure and heat, etc. In the fixing unit, a pressing member and a fixing member are positioned while contacting each other and forming a fixing nip (i.e., a nip) between the pressing member and the fixing member. Such a pressing member and the fixing member are composed of either a pair of opposed rollers or belts or a combination of rollers and/or belts. For example, among various types, a roller type-fixing unit forms the fixing nip by pressing the fixing roller against the pressing roller, with the fixing roller containing a heat source, such as a halogen heater, etc. Thus, when a recording medium bearing an unfixed toner image on its surface is conveyed through the nip formed between a pair of rotary members (e.g., a fixing roller and a pressing roller) currently rotated, heated, and pressed against each other at the time, the roller type fixing unit subsequently applies the heat and pressure generated between the fixing roller and the pressing roller to the recording medium to melt and fix the unfixed toner image borne on the recording medium.
Recently, in accordance with a growing demand for saving energy and shortening a wait time required for heating a fixing unit (i.e., a warm-up time, a time to first print or the like), a so-called on-demand type fixing unit that decreases its own heat capacity by employing an endless belt, such as a belt unit, a thin film, etc., instead of a roller (i.e., a fixing roller), thereby upgrading efficiency of heat transfer to a recording medium while dramatically shortening the waiting time is widely adopted. As this kind of the fixing unit, there is, for example, a conventional system like that disclosed in JP-2008-158482-A, which includes a belt unit and a securing member (e.g., an opposite member) that borders and slides on an inner circumferential surface of the belt unit while being pressed against a rotary pressing member (e.g., a pressing roller) through the belt unit to form a nip between the belt unit and the rotary pressing member. Such a conventional system fixes a toner image on a recording medium by conveying the recording medium into the fixing nip as already known. Such a fixing unit of the system is generally provided with a heat transfer member (e.g., a heater) disposed either close to or contacting the inner circumferential surface of the belt at a position other than the nip. The heat transfer member thus additionally serves as a belt guide sometimes in this point of view.
In this type of the fixing unit, movement of the belt at inlet and outlet sides of the nip in a conveying direction of the recording medium is technically important, especially, in view of transportation of the recording medium. For example, when a positioning relation between the securing member and the heat transfer member deviates from a desired target due to deformation of the heat transfer member or the like, the following problems likely to occur.
First, when the heat transfer member protrudes at the inlet side of the nip toward the rotary pressing member from a nip formation surface specifying a nip, entry of the recording medium to the nip worsens, possibly wrinkling the recording medium. In addition, a trailing end of the recording medium flutters, so that an image on the recording medium may be rubbed by surrounding members surrounding the nip.
By contrast, when the heat transfer member protrudes at the outlet side of the nip toward the rotary pressing member from the nip formation surface, the belt excessively winds around the rotary pressing member at a larger angle than is usual, thereby seeming to increasing a nip width (i.e., an apparent nip width) as a result. Consequently, problems, such as deterioration of image quality due to excessive heating of the toner image, damage to or cuts on the belt caused when a recording medium separator contacts the belt at downstream of the nip, etc., are likely to occur.
Conversely, when the heat transfer member is extremely indented at the outlet side of the nip from the nip formation surface, a gap between the belt and a separator that separates the recording medium from the belt excessively increases more than expected, so that the recording medium likely enters the gap and becomes unable to be ejected from the gap.
In this respect, not to cause the heat transfer member (i.e., a heater) to deform and thus deal with such problems, JP-2010-96782-A discloses a fixing unit in which a heat transfer member has an opening to arrange a securing member that clamps and fixes the opening not to widen with a pair of stays. With such a conventional technology of JP-2010-96782-A, since deformation of the heat transfer member can be almost prevented by fixing the opening of the heat transfer member, movement of the belt can be stabilized actually. However, in the conventional technology of JP-2010-96782-A, since the opening is secured by pinching the opening with the pair of stays, the number of parts and that of assembly steps significantly increase, thereby again, increasing heat capacity of component parts, accordingly. As a result, it is difficult to both shorten the heating time and save energy as well. Further, since the pair of stays is provided in the opening, displacement (i.e., layout) and shape of load bearing members are limited. Especially, a length of the load-bearing unit (i.e., a reinforcing unit) that bears the load is limited in a loading direction of the load. Specifically, such limitation is generally disadvantageous when bearing a relatively larger nip load.